Pluripotent stem cells have the ability to differentiate into all types of cells (endodermal, ectodermal and mesodermal origin) and therefore have the potential to regenerate any kind of tissue with proper manipulation. Currently, two gold standard pluripotent stem cells are known, embryonic stem cells (ESCs) and induced pluripotent stem cells, which are reprogrammed adult somatic cells (iPSCs).
ES cells have unequivocally taken center stage in the field of stem cell research. ES cells exhibit the potential to treat a plethora of previously irreversible disorders through their capacity to generate tissues and thus to revolutionize regenerative medicine (1-3). However, evidence has since emerged that ES cells exhibit high rates of immunorejection upon transplantation and form teratomas as a result of their unbridled proliferation (4). In conjunction with debates surrounding the bioethical issues concerning the usage of human embryos, this teratogenic propensity precludes the practical application of ES cells in regenerative medicine.
Addressing the ethical dilemmas surrounding the use of ES cells for cell therapy, iPS cells became of interest in the stem cell field (5-6). iPS cells have the capacity to re-program, through an intricate mechanism involving the induction of the so-called “Yamanaka factors,” including Nanog, Oct 3/4, Sox2, c-Myc and Klf4, which subsequently became the characteristic markers that establish pluripotency: the ability to self-renew and generate cells from the three germ lines and thus form teratomas (7-9). Though iPS cells resolve concerns of immunorejection because they can be generated from a patient's own, or autologous, cells, as well as the ethical issues that hinder the use of stem cells extracted from human embryos, the production of teratomas upon transplantation as a result of unbridled cell proliferation and extremely low survival rate of both iPS and ES cells upon reintroduction to the host organism, impede the translational use of these cells (10-12). Furthermore, it has also been found that mature iPS cells possess an epigenetic memory, defined by the remnants of posttranslational histone and DNA modifications, preventative of entirely successful reprogramming, often restricting their physiological function to that of a cell within the same lineage as the original stem cell source (13-15). Investigators have made attempts to address these issues, but to little avail (16-17). Despite excessive monetary and temporal efforts devoted to the study of both ES cells and iPS cells, there has been little progress made in overcoming the hurdles facing these stem cells and their use for cell therapy.
Other, non-reprogrammed pluripotent stem cell populations have caught the attention of the scientific community as an alternative to ethically contentious ES cells and genetically modified iPS cells. However, though multiple populations of adult stem cells have been put forth, many have faced a great deal of suspicion due to irreproducibility. Isolated from bone marrow, multipotent adult progenitor cells (MAPCs), both pluripotent and non-tumorigenic, were reported to contribute to chimeric offspring when injected into a mouse model and to regenerate damaged tissue in vivo (18-19). Human marrow-isolated adult multilineage inducible (MIAMI) cells and very small embryonic-like stem cells (VSELs), isolated from umbilical cord blood in addition to bone marrow, were soon to follow, exhibiting similar pluripotent and non-tumorigenic properties. Like VSELs, unrestricted somatic stem cells (USSCs), isolated from umbilical cord blood, are reportedly pluripotent but lack the classic pluripotent stem cell marker expression (20). These adult pluripotent stem cell lines have all been publically flagged for further investigation and reproduction, or in the case of VSELs, negated entirely. Stimulus-triggered acquisition of pluripotency (STAP), characterized by exposing splenic CD45+ lymphocytes to acidic conditions followed by incubation with leukaemia inhibitory factor (LIF), has recently been described as a method of bestowing pluripotency upon somatic cells (21). However, STAP cells form teratomas, hindering their clinical application. STAP cells are currently under investigation to determine the overall validity of the published results as well as the mechanism behind their reprogramming.
A population of human pluripotent stem cells with a high post-transplantation survival rate that does not undergo teratogenesis in vivo can facilitate treatment of many disorders affecting human beings. Recently, a group of researchers from Tohoku University, Japan, isolated and cultured a stem cell population isolated from skin and bone marrow with pluripotent characteristics (22-23). These cells named Muse cells (Differentiating Stress-Enduring Multilineage Cells) can be created in vitro under cellular stress conditions. Muse cells grow while forming cell clusters. Muse cells have been characterized as mesenchymal stem cells that have the ability to express a set of genes associated with pluripotency.
Furthermore, Muse cells can differentiate into endodermal, ectodermal, and mesodermal cells both in vitro and in vivo. More importantly, unlike ESCs and iPSCs, when Muse cells are transplanted (by local or i.v. injection) into immunodeficient mouse models for tissue regeneration, Muse cells integrate into damaged skin, muscle, or liver and regenerate new tissue. Muse cells do not undergo tumorigenic proliferation, and therefore would not be prone to produce teratomas in vivo, nor do induce immuno-rejection in the host upon autologous transplantation (22-23). Furthermore, in contrast to iPSCs, Muse cells do not require introduction of exogenous genes for their pluripotency. In addition, Muse cells are shown to home into the damage site in vivo and spontaneously differentiate into tissue specific cells according to the microenvironment to contribute to tissue regeneration when infused into blood stream (22).
Muse cells have the potential to make critical contributions to tissue regeneration, but are hindered due to difficulties associated with extraction of bone marrow stromal cells and human skin fibroblasts, and time-consuming purification methods including cell sorting, cell cloning by limiting dilution, long periods of cell culture which lead to a final production of only 1,000,000 Muse cells from of bone marrow stromal cells or human skin fibroblasts after SSEA3 cell sorting and one month of cell expansion. There thus remains a need for isolation of non-tumorigenic pluripotent cells as a source of tissue regeneration, and for improved methods of isolating such cells.